1. Field of the Invention
The present invention relates generally to control of audio amplifier outputs, and more specifically, to techniques for disabling and enabling audio amplifier outputs by modulating between enabled and disabled states.
2. Background of the Invention
Audio amplifiers typically exhibit pops due to changes in output levels when power is applied and the output is otherwise disabled or enabled. The output offset can be due to internal offsets in the amplifier and/or due to offset present in the input signal that is passed to the output of the amplifier. With present-day headphone amplifiers having very low signal output requirements due to highly efficient headphone designs, such as in-ear headphones, even a relatively small step in the output voltage of the headphone amplifier can generate an audible pop.
One way in which pops have been reduced or eliminated in previous amplifier designs is by controlling the bias of the transistors in the amplifier output stage according to a ramp waveform, such as that provided by the charging of a capacitor with a bias reference current that is internally mirrored to set the quiescent current levels in the amplifier output stage.
However, an amplifier that responds to such analog control is generally more complicated than a digital approach that merely disables and enables the output stage by completely disabling/enabling the transistors in the amplifier output stage and in some applications and circuit topologies, a binary control signal may be the only option for controlling the enable/disable state of the amplifier output.
Therefore, it would be desirable to provide techniques for disabling and enabling an audio amplifier output stage in which binary output stage control may be used while avoiding the generation of audible pops.